While it is illegal for senators to use campaign funds for their own personal expenses, there are many avenues available to Baucus for spending the funds, such as contributing to other politicians or charities — even charities set up by the candidates themselves as former Rep. Allen West has done.

Though Election Day isn't until November of 2014, the Montana Senate race had already drawn its first instance of outside spending. Last week, Sunlight's Ad Hawk picked up a spot from the National Association for Gun Rights urging voters to tell Baucus to vote against the Manchin-Toomey amendment to expand background checks on gun transfers. (He did, becoming one of only four Democrats to oppose the measure, which failed on what President Barack Obama called "a pretty shameful day for Washington.")

As the head of the Finance Committee, Baucus has been at the epicenter of tax and other important monetary legislation — and financial industries have courted him with millions in campaign contributions because of it. Since 1989, securities and investment groups have given almost $1.8 million to Baucus, while commercial banks have contributed more than $575,000 and miscellaneous financial interests have chipped in over $500,000.

Instrumental in the congressional effort to craft Obama's health care law, Baucus received millions more from the medical industry. Big insurance companies doled out more than $1.5 million, health professionals over $1.2 million, pharmaceuticals almost $1 million, hospitals nearly $800,000 and health services/HMOs gave over $780,000. Sunlight reported the influx of cash Baucus received for his version of healthcare reform as it happened in 2009.

Among his top donors are financial and insurance titans like Goldman Sachs, AIG and New York Life, each of which gave almost $100,000 over the course of his career.

Baucus' office is also known for its powerful staff alumni association: a broad constellation of tax lobbyists who represent interests from oil companies to telecom businesses. Currently, there are 34 former employees of Baucus' office who have gone through the revolving door, and an analysis by the New York Times chronicled how 28 of those who have worked for Baucus since he became Finance Chair in 2001 have lobbied on tax issues during the Obama administration. These lobbyists, including former chiefs of staff like Zak Anderson — now a director of federal government relations at BNSF Railways — will play a major role in crafting tax reform legislation.

But the network of lobbyists who have come from Baucus' office doesn't end there, as several more have influenced Congress during the passage of healthcare. In 2009, an analysis by Sunlight revealed that five of Baucus' former staffers worked for a total of 27 different organizations that are either in the health care or insurance sector, such as Amgen and GE Health Care. Former chiefs of staff Jeff Forbes and David Castagnetti fall into this group, and both still hold senior positions in high-powered lobbying shops today.

The dollars Baucus has raised over the course of his prolific fundraising career far outnumber the 1 million constituents he has back home. Sunlight's Party Time, shows Baucus holding more than 30 events since 2008. Splitting his time between Capitol Hill hot spots and the rugged terrain of Big Sky in Montana, he routinely attended $5,000 receptions hosted by tax and healthcare lobbyists.